Sounding Off on Alarm Audibility

Today, much of the hype on security systems is about the world of digital technology. This month, we are going to take a look at some important aspects of sound technology such as system audibles like sirens, speakers and sounders. 

The proper operation of these devices is an integral part of any security system. Yet audio technology is often not well understood or applied properly. A better understanding of sound technology is also needed to help comply with industry standards such as NFPA 72, NPFA 371 and OSHA. 

Now Hear This!
Let’s start out with a primer on the physics of sound. As we learned in high school physics, sound is generated by an object’s movement, which, in turn, creates a sound pressure wave that is transferred through our atmosphere. The wave travels at a speed of 770 mph until it reaches a human ear, another object or electronic sensor such as a microphone. Temperature, humidity and objects that reflect or absorb can easily influence the performance of sound wave transmission. 

The measurement unit for sound loss and gain is the decibel (dB). It is a logarithmic unit used for the ratio of, in this case, the sound level of system audible devices to the lowest human hearing level (0dB). Logarithmic measurements can often be confusing to calculate. However, the decibel scale is handy for the technician in estimating the sound level, and/or gain of sound. This dB measurement is used in a variety of standards readings for the sound levels of security audibles. 

If a person were to move away from a sound source, such as a siren, every doubling of distance would cause the sound level to drop off a factor of 4dBA or 6dBA (decibel “A” weighting scale). This is due to the area influenced being four times as large.

We have an estimating guideline where ±3dB either doubles or halvesthe sound power levels. An example is when an electronic siren is rated by the manufacturer at 118dB @ 10 feet and another is rated at 120dB, the second siren sound is only +3dB higher but is approximately twice as loud. 

The above dB calculation guidelines are in a perfect world or lab, which, in real life, is not going to happen. So real-world applications may vary due to the environmental conditions. It has been said that for every 8dB to 10dB change, the human ear perceives either a doubling or halving of the sound. 

I can remember a residential customer who had an exterior siren installed in an attic vent and complained because their neighbors could not hear the siren and wanted the faulty siren fixed. It so happened that the siren sounded in the winter, when cold temperature, snow, humidity and the neighbors’ insulated storm windows caused a considerable reduction in sound level. The initial installation had taken place in the mild summer months. 

To make things worse, the customer, and probably the security dealer, was not aware of the directional sound radiation patterns of electronic sirens. In this case, an additional siren on the opposite side of the house and louder sirens of just a few dB would have helped this situation. 

Always check to see who is expected to hear alarm audibles. Also be aware that certain higher frequencies, such as some keypad sounders, may be harder for seniors to hear. You may need to add auxiliary annunciation. 

An interior application could be acoustical sound detectors that are installed and tested in a new construction environment and then do not function properly later when carpets, drapes and furniture are installed. The impact sounds are absorbed. Sound sensors also might false alarm due to high ambient sounds when the system is armed. A final test and reading should be done after complete occupancy. 

Follow Basics of Sound Safety
Let’s start out with a very important sound area — hearing safety. The Occupational Safety and Health Administration (OSHA) has noise level guidelines for your safety when operating equipment such as impact drills or just being in a construction environment. Sounds louder than 80dB are considered potentially dangerous. 

These sounds are measured on a ratio of higher noise level to period of exposure during an eight-hour work period. Wear hearing protectors over your ears when using loud equipment. If you have to shout to talk to someone, the area is too loud. 

A basic word of caution is warranted for working on exterior alarm sirens while up on a ladder. Make sure to first disconnect the siren output at the alarm panel. This suggestion sounds simple, but I have almost been blown off a high ladder from accidental alarm activation when working on a siren. 

Meeting NFPA 72 Requirements
Fire code requires that alarm sound levels be 15dBA above the ambient sound level or 5dBA above the maximum sound level having a duration of 60 seconds, whichever is greater, measured 5 feet from the floor. If you do not test for this and the AHJ does, the owner may not get his Certificate of Occupancy (CO). Learn to test with a sound level meter. 

Another warning about fire sound level requirements: You may be required to have relays cut out sounding devices, such as background music or entertainment sound systems, when an alarm sounds. Large public areas such as malls can present a big challenge with controlling ambient sound during an emergency. Remember these are life-safety issues.

NFPA 731 Includes a Sound Test
Even the newly evolving NFPA 731 standard has a reference to sound levels. It states, “Audio sensors using a sound level meter designed, constructed and calibrated in accordance with ANSI S1.4, Specification for Sound Level Meters, determine the average ambient sound does not exceed 65dBA during the period the intrusion detection system is armed. The area covered by a single detector shall not exceed the area of coverage specified by the detector manufacturer. Utilizing the method recommended by the manufacturer, test the operation of the system.”

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